Prior methods of motion control were based on fixed "servo curve tables". The "servo curve table" does provide a quick "lookup" algorithm that allows the single microprocessor code timing to complete 8 axis of real time position feedback comparison with the table elements in less than a millisecond. The lookup table consists of 127 error distance ranges for acceleration control and 127 error distance ranges for deceleration control. Each error distance range table element contains an associated velocity reference value. The change in error distance controls the change in velocity thus controlling the acceleration and deceleration rate.
The error distance values are in units of encoder feedback counts which can be translated to linear or rotary units such as inches or degrees. The reference command velocity is in digital units that is converted to an analog signal by a digital to analog converter. This analog signal is input to a suitable power amplifier that supplies power amplification and closed loop velocity and current control of the servo actuator.
Motion control of high performance servo axis using this method has many disadvantages that are improved upon with advanced digital motion control.
The programmability of multiple acceleration and deceleration rates are required because of sensitive force-mass applications such as picking, transporting, rotating and placing various size surface mount components with servo drive actuators. The memory space requirements to store all "servo curve table" rate requirements for up to eight different axis is limited or impossible with a single microprocessor controller.
There is no digital reference time control of the motion profile, thus performance accuracy and repeatability is limited by the quality of the analog velocity loop, variations in electrical tolerances and mechanical system tolerances. Applied to a system this affects machine cycle rate, motion force accuracy, jerk force variability, position overshoot, and position settling time.